Power Unit

LDMOS Explain

 

LDMOS (laterally diffused metal oxide semiconductor [1]) transistors are used in microwave/RF power amplifiers. These transistors are often fabricated on p/p+ silicon epitaxial layers. The fabrication of LDMOS devices mostly involves various ion-implantation and subsequent annealing cycles.[1] As an example, the drift region of this power MOSFET is fabricated using up to three ion implantation sequences in order to achieve the appropriate doping profile needed to withstand high electric fields.

Silicon-based LDMOS FETs are widely used in RF power amplifiers for base-stations as the requirement is for high output power with a corresponding drain to source breakdown voltage usually above 60 volts.[2] Compared to other devices such as GaAs FETs they show a lower maximum power gain frequency.

Testimony of Ampleon USA Inc:

The next test of ruggedness is with a controlled load mismatch through all phases, enabled by the test fixture now in view. The mismatch unit is connected to a network analyzer. Through a network control we can set it to work through all phases of the Smith chart. This mismatch unit will create voltage standing waves with ratios from 55: 1 up to a maximum of 125: 1. With the normal situation, standard, 50 volts 1200 watts into a 50-watt load, so at this moment the amp is operating at full power. The mismatch unit now replaces the dummy load connected to the output of the amp and the test set up is powered up again. The mismatch unit is now varied to expose the LDMOS transistor to a range of extreme mismatch conditions and positions on the Smith chart. The extreme conditions vary from VSWR from 55: 1 to 125: 1. Now we show the transistor is still alive by going back to the original load and showing the 1200-watt output power. It still works. Would you ever treat a transistor worse than this? Check out the next test.

Even though the transistor is normally used at 50 volts, we test at 55 volts to challenge the extremely rugged LDMOS. We set up the transistor at 55 volts 1200 watts into 50 ohms. We disconnect the output load creating a very unfriendly open circuit situation. We power up the transistor to 1200 watts. We screw the short to the output of the amp while the transistor is powered up. We go back to the 50-ohm load and power up the transistor to 1200 watts and we see that it again survives and works perfectly. This is what we mean by extremely rugged. This transistor seems to be unbreakable.

PA Diagram

Input Protection

The Achilles heel of the LDMOS device is the gate. Let’s say that to avoid a car accident, it is best to never leave home. Basically, this is the input protection. If the input attenuator is 16 dB, 100 watts never breaks the LDMOS and 50 watts will be sufficient for maximum power. In addition, suppressor diodes SM6T15A Di, D2, D3, D4 are incorporated. The SM6T Transil series has been designed to protect sensitive equipment against electrostatic discharges according to IEC 61000-4-2 and MIL STD 883, method 3015, and electrical over stress according to IEC 61000-4-4 and 5. These devices are more generally used against surges below (10/1000 µs). video

All IDC connections are compatible with current KM3KM designs.

Board Connections

Measurements

Intermodulation (IM) or intermodulation distortion (IMD) is the amplitude modulation of signals containing two or more different frequencies, caused by nonlinearities or time variance in a system. The intermodulation between frequency components will form additional components at frequencies that are not just at harmonic frequencies (integer multiples) of either, like harmonic distortion, but also at the sum and difference frequencies of the original frequencies and at sums and differences of multiples of those frequencies.

For IMD3 measurement I relied on ICOM 7300 (SDR Radio) and Telepost LP500 Station Monitor (good results).Video

Expert 1K-FA IMD3 Video

Trapezoid pattern

Top half of a trapezoidal display plots exciter output vs. amplifier output and displays a triangular image that easily shows when the output of the
amplifier becomes nonlinear, ie. flat tops and no longer provides the same gain as at lower power. ½ Trap shows more detail than normal trapezoid.

For Trapezoid pattern measurement I relied on ICOM 7300 (SDR Radio) and Telepost LP500 Station Monitor (good results) Video

Expert 1K-FA Trapezoid pattern Video

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